Furnace for treatment of wax-bonded sinterable preforms



1968 H. VOLLMER ETAL 3,361,864

FURNACE FOR TREATMENT OF WAX-BONDED SINTERABLE PREFORMS Filed Jan. 5,1965 gm 4. array/v52.

United States Patent 3,361,864 FURNACE FOR TREATMENT OF WAX-BONDEDSINTERABLE PREFORMS Helmut Vollmer, Heiligwies, and Kurt Huchler,Balzers, Liechtenstein, assignors, by mesne assignments, to The BendixCorporation, Detroit, Mich, a corporation of Delaware Filed Jan. 5,1965, Ser. No. 423,509 Claims priority, application Switzerland, Jan. 9,1964, 237/64 2 Claims. (CI. 13-27) ABSTRACT OF THE DISCLOSURE A furnacefor the treatment of wax-bonded sinterable preforms. The furnaceconsists of an evacuated housing containing a sealed container intowhich the sinterable preforms are inserted. The first passage leads fromwithin the container to the space between container and housing. Asecond passage leads from the space between container and housingexternally to associated condensing and pumping mechanism.Externally-controllable valve means are operative to selectivelyinterconnect the first and second passages. The arrangement permits,without handling of the wax-bonded sinterable preforms, first the waxremoval and condensing in the associated mechanism, and secondly, vacuumsintering of the preforms.

In the sintering technique it is useful to press the powdered rawmaterial with the use of a binding material into preforms, which thenare treated in a furnace at correspondingly higher temperatures tobecome the end prod uct. Various materials are used as the binding meansamong which are waxlike substances. Before or during the real sinteringprocess the binding material must be removed. One of the known processesin the case of use of wax as the binding material involves thefollowing: after the pressing of the preforms to attain a sufficientlypermanent shape, they are heated to a temperature sufficient to vaporizethe wax so that the wax vapor can be pumped off or trapped with cooledcondensers. The use of special vacuum dewaxing furnaces with theirauxiliary equipment is costly, time consuming and unsatisfactory. Italso possesses the disadvantage, that by dewaxing before thesintering-the binding is more or less generally removed from thepreforrns and they lose their durability and on account of this theymust be handled with the greatest of care in removal from the dewaxingfurnace and loading into the true sintering furnace.

Up to the present time the dewaxing and sintering processes could not becarried out in the same furnace, because with the present furnaceconstruction the wax vapor removed from the sintering material has ledto a contamination and dirtying of the whole inside of the furnace andthe connected pump, which becomes intolerable for succeeding sinteringoperations.

It is the object of the present invention to overcome the abovementioned difficulties and to be able to carry out the dewaxing andsintering processes in one and the same vacuum furnace.

The furnace according to the invention for the treatment of wax-bondedpreforms to be sintered consists of a closed furnace housing with anevacuation connection, a container situated within it and heating meansfor the exposure and heating of the preforms to be dewaxed and sinteredand is characterized in that the aforementioned container on the insideof the evacuable furnace housing is built with an evacuation openingwhich is connected to a wax vapor condenser by a second separateevacuation pipe. One example of the carrying out of the invention isrepresented in the accompanied schematic drawing in which:

FIGURE 1 is a schematic view of a furnace embodying the presentinvention, and

FIGURE 2 is a fragmentary view similar to FIGURE 1 but showing anotherembodiment of the present invention.

Referring now to the drawings, numeral 1 designates a closed furnacehousing which can be evacuated by means of a high-vacuum pump (notshown) through the evacuation connection 2. Inside the furnace housingare arranged two containers 3 and 4 and a heating arrangement for thecontainer 4 such as an induction heating coil 5 surrounding thecontainer 3. Container 4 is located within container 3 and may be eithermetal or graphite depending on temperature requirements. Container 3 ismade of quartz or quartz-like material. The container 4, as shown in thedrawing, consists of a graphite cup 6 with a removable cover 7 embeddedin carbon grit 8 which surrounds the sinter material on all sides. Thecarbon grit 8 above the cover 7 may be loosely packed to facilitateremoval of the carbon grit 8 and allow insertion and removal of thesinter material. The container 4 has an opening 11 in the bottom whichis connected to a second evacuation connection 12. Conduit 13 extendsfrom connection 12 through the wall of the furnace housing in a vacuumtight manner to the wax condenser 18 which is adapted to be heated bywell known means (not shown). A shaft 15 is disposed within conduit 13and is connected to a small plate valve 16 to open and close said valveand thereby respectively connect conduit 12 and the interior ofcontainer 4 to the space 9 of the evacuable furnace housing 1 or toconduit -13 as desired. The shaft 15 is likewise carried out from thefurnace housing in a vacuum tight manner and is provided with ahand-wheel 1'7. The valve plate 16 and the hand wheel 17 are shown inthe closed position of the valve in FIGURE 1. The open position is shownby dotted lines. The possibility of the connection of the container 4with the space 9 permits the bringing of the sinter material containerto high vacuurn before the beginning of sintering, with the pumpsconnected at 2. The vacuum pump 19, connected to the pipe 13 and to thecondenser identified as a whole as 18, then only serves for the separatepumping off of the wax vapor during dewaxing. Under other conditionspump 19 can be used with suitable capacity to evacuate container 4.

The condenser '18 is of a well known construction and consists, in theexample given, of a vessel 20, in which are arranged coolable baffieplates 21 for the condensation of the wax vapor. The centrally arrangedcarrier 22 for the baffle plates is hollow and suitable cooling fluidcan be circulated through it.

The removal of the wax vapor from the sinter material container 4 andthe permanent gases evolved during the dewaxing process takes placethrough the pipes 12, 13 and 23. The outlet from the condenser leadsthrough the pipe 24 to the pump 19. The entrance to and exit from thecondenser 18 may be shut-off by valves 25 and 26 respectively.

The described furnace is suitable for dewaxing as well as sintering. Thesinter material is introduced into the container 4 which is then closed.The container and the furnace housing 1 are evacuated through the openvalve 16 and the pump connected at 2. The valve 16 is then closed andthe sinter material brought to the dewaxing temperature by energizingthe heater. The evolving wax vapor is removed through the pipe 13 andcondensed in condenser 18. The non-condensable gases, which emerge atthe same time from the container 4, as was previously mentioned, areremoved by the pump 19. The induction coil 5 may be connected to anelectrical A-C current 3 source, not shown, through vacuum tightconnectors, out of the plane of the drawing and therefore not shown,which are known to those skilled in the art.

It is often advantageous during the dewaxing to maintain an overpressureof an auxiliary gas in the furnace which does not disturb the carryingout of the dewaxing process. The auxiliary gas can, for example, beargon, nitrogen or hydrogen and may be admitted through the evacuatedconnection 2. Overpressure during the dewaxing process hinders thepossible escape of the wax vapor into space 9 through the lack ofthightness of the sinter material container 4. An auxiliary gasatmosphere in the furnace may also be used in the sintering processitself to reduce the vaporization of components of the sinter materialat the higher temperature used.

After the carrying out of the dewaxing it is not necessary to remove thematerial from the container. In most cases the condenser can be simplyshut off from the furnace by means of valve 25. The valve 16 is openedand the space 9 is well as the sinter material container .are brought tohigh vacuum and after increasing the heat the sintering is carried outin the usual manner.

The necessary reduced pressure adjusts itself after a short time to thesintering process and lies somewhere between 10 to torr.

The operating technique is especially advantageous if two or more of thesintering furnaces according to the present invention can be combinedinto a single operating unit. The charge of one furnace may be dewaxedwhite the other is being sintered. For this purpose the pipe 23 inFIGURE 1 may be removable from the furnace.

A further possibility for such manifold arrangement is shown in FIGURE2. The same parts are designated by the same reference numbers as inFIGURE 1. The connection between pipe 12 from the bottom of the sintermaterial container 4 and the condenser 18 is different from that inFIGURE 1.

In this embodiment the connection comprises a flexible rubber tube 32and with its connection piece 30 is detachably connected to pipe 12through a shut-off valve 31 secured to the furnace housing 1. Tube 32 isretractable from the evacuated furnace to the position (30732) as shownby the dotted lines in FIGURE 2. Tube 32 may be heated in practice bysuitable well known means (not shown).

The shut-off valve 31 is of known construction and will therefore not befurther described. An 0 ring gasket seal 33 prevents the entrance ofatmospheric air into the furnace while the valve 31 is opened for theinsertion or removal of the evacuation tube. After drawing back the pipeor tube 32 from the furnace the valve 31 can be again closed and thetube detached from the furnace. The condenser with its attached pumps isthen ready for use in dewaxing of sinter material in other furnaces ofthe same type.

Wax in the sense of the present invention will be understood to meanevery principle binding material of the higher hydrocarbons, which canfind use in the sintering technique, especially in the production ofhard metal preforms.

We claim:

1. A furnace for the treatment of wax-bonded sinterable preformscomprising: a housing; an enclosed container adapted to hold sinterablepreformsin the interior thereof and in said housing, defining a spaceadapted to be evacuated intermediate said housing and said container;induction means for heating said container; a first evacuationconnection opening on one end into the interior of said container and onanother end into said space intermediate housing and container; a secondevacuation connection opening into said space on one end and extendingexterior of said housing; valve means controllable externally of saidhousing for selectively interconnecting said first and second evacuationconnections, condenser means exterior of said housing means and conduitmeans interconnecting said second evacuation connection and saidcondenser means.

2. The furnace of claim 3 wherein said conduit means is detachablyconnected to said second evacuation connection and a shut-off valve isconnected to said second evacuation connection to close said connectionwhen said conduit means is detached.

References Cited UNITED STATES PATENTS 2,789,153 4/1957 Mark 133lRICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Assistant Examiner.

1. A FURNACE FOR THE TREATMENT OF WAX-BONDED SINTERABLE PREFORMSCOMPRISING: A HOUSING; AN ENCLOSED CONTAINER ADAPTED TO HOLD SINTERABLEPREFORMS IN THE INTERIOR THEREOF AND IN SAID HOUSING, DEFINING A SPACEADAPTED TO BE EVACUATED INTERMEDIATE SAID HOUSING AND SAID CONTAINER;INDUCTION MEANS FOR HEATING SAID CONTAINER; A FIRST EVACUATIONCONNECTION OPENING ON ONE END INTO THE INTERIOR OF SAID CONTAINER AND ONANOTHER END INTO SAID SPACE INTERMEDIATE HOUSING AND CONTAINER; A SECONDEVACUATION CONNECTION OPENING INTO SAID SPACE ON ONE END AND EXTENDINGEXTERIOR OF SAID HOUSING; VALVE MEANS CONTROLLABLE EXTERNALLY OF SAIDHOUSING FOR SELECTIVELY INTERCONNECTING SAID FIRST AND SECOND EVACUATIONCONNECTIONS, CONDENSER MEANS EXTERIOR OF SAID